1. Field of the Invention
The present invention relates to a feedback type variable gain amplifier, which is specifically suitable to an amplifier used for high frequency band, UHF or higher band.
2. Related Background Art
FIG. 1 shows a circuit diagram of a conventional feedback type variable gain amplifier.
The conventional feedback type variable gain amplifier is provided with an amplifier AMP31, and a gain control circuit GC31 which is a feedback circuit coupled to the amplifier AMP31 and controls a value of the gain. The amplifier AMP31 includes a series-connected high frequency cutoff inductor L31 and amplifying field effect transistor FET31 across a power-supply potential terminal Vdd and a ground potential terminal GND. The gate of the transistor FET31 is used as a signal input terminal Vin, and the drain is used as a signal output terminal Vout. The gain control circuit GC31 includes a series-connected gain control field effect transistor FET32 and direct-current cutoff capacitor C31 across the signal output terminal Vout and the signal input terminal Vin. Furthermore, a resistor R31 connected across the drain and the source of the gain control field effect transistor FET32.
In this conventional feedback type variable gain amplifier, the resistor R31 maintains levels at the drain and the source of the transistor FET32 to the same potential, the capacitor C31 isolates levels at the gate of the transistor FET31 from the power-supply potential Vdd. Furthermore, by varying a voltage Vgc applied to a gain control terminal G32, which is the gate of the transistor FET32, the transistor FET32 and the resistor R31 function as a variable resistor having a resistance value R(Vgc).
FIG. 2 shows a circuit diagram of an equivalent circuit of the gain control circuit GC31 in the conventional feedback type variable gain amplifier shown in FIG. 1.
The equivalent circuit of the gain control circuit GC31 is expressed by a series connection of a variable resistor R41 and a direct-current cutoff capacitor C41. And as described above, since the transistor FET32 and the resistor R31 of the gain control circuit GC31 function as a variable resistor, an impedance Z of the gain control circuit GC31 is represented as follows: EQU Z=R(Vgc)+1/(j.omega.C)
where, R(Vgc) is a variable resistance value between the drain and the source of the transistor FET32, and R(Vgc)&lt;&lt;R31.
A phase difference of a voltage to a current flowing through the circuit is represented as follows: EQU .omega.=tan.sup.-1{-1/(.omega.CR)} &lt;0
In the conventional feedback type variable gain amplifier, the direct phase difference between the input voltage and the output voltage is 180.degree.. However, as the frequency increases, the phase difference decreases because of the shifting of the phases of the input voltage and the output voltage. The range of the phase of the impedance Z of the gain control circuit GC31 is -90.degree.&lt;.theta.&lt;0.degree.. The phase of the signal fedback to the input terminal Vin of the amplifier AMP31 via the gain control circuit GC31 has a value which is obtained by adding the phase difference of the gain control circuit GC31 to the phase difference of the amplifier AMP31.
In order to obtain an ideal negative feedback for the amplifier, it is desirable to feedback the signal to the input terminal of the amplifier with the phase difference 180.degree.. However, in the conventional feedback type variable gain amplifier shown in FIG. 1, the decay of the gain is small because the phase difference of the signal fedback to the input terminal of the amplifier can not be 180.degree., especially in high frequency band, UHF or higher band. Consequently, there is a problem that it is difficult to expand the range of the controllable gain.